This invention generally relates to alignment devices and methods and more particularly relates to a self-locking alignment pin and method for aligning a nuclear fuel assembly with respect to a core plate disposed in a nuclear reactor pressure vessel, the alignment pin being connectable to the core plate exclusively from below the core plate.
Nuclear reactor vessel control rods are controllably lowered vertically through the upper core plate into the thimble tubes of the fuel assemblies and also raised vertically out of the thimble tubes through the upper core plate. Therefore, it is important that the fuel assemblies be accurately aligned relative to the upper core plate in order to allow accurate and free passage of the control rods into and out of the fuel assembly thimble tubes.
To reference the fuel assembly to the upper core plate, the core plate typically has protruding alignment pins oriented parallel to the axes of the fuel rods, control rods and thimble tubes. The alignment pins connected to and protruding downwardly from the upper core plate mate with openings in the top nozzle of the fuel assembly, thereby fixing the positions of the fuel assembly thimble tubes and control rods relative to the upper core plate.
However, the alignment pins are sometimes damaged (e.g., bent) during fuel assembly servicing. Although slightly bent alignment pins can be satisfactorily straightened, some alignment pins may be bent severe enough to cause their replacement. Bent alignment pins are undesirable because bent alignment pins can adversely affect the ability to control the reactor core, due to the resulting misalignment of the control rods and the thimble tubes for receiving the control rods. Therefore, in order to avoid the risk of such misalignment, the alignment pins are routinely checked, and replaced if necessary.
A certain amount of space is needed to accommodate the tools and video cameras needed to conduct pin replacement operations. In this regard, part of the upper internals of the reactor, in the region above the upper core plate, is removed to gain access to clamping nuts of the alignment pins, which clamping nuts are only accessible from above the upper core plate. However, removal and replacement of parts of the upper internals is time consuming and may cause damage to such structures. Therefore, it is desirable to provide an alignment pin replacement technique which is conducted entirely from below the upper core plate thereby obviating the need to remove and replace the upper internals.
Moreover, prior art alignment pins engage the core plate by an interference fit with a bore formed in the core plate to assist locking the alignment pin to the core plate. That is, the alignment pin has a diameter slightly larger (e.g., 0.001 inch) than the bore in the core plate that receives the alignment pin. Therefore, after the clamping nuts are removed, such an alignment pin must be drilled out of the hole from beneath the core plate and the hole is re-sized in order to receive a replacement alignment pin. However, it would be preferable if replacement of the alignment pin were not to require the time-consuming process of drilling-out the alignment pin and resizing the bore.
However, alignment pins for aligning a nuclear fuel assembly to an upper core plate of a nuclear reactor and that are replaceable entirely from below the core plate are known. A guide pin aligning a top nozzle of a nuclear fuel assembly to an upper core plate of a nuclear reactor and that is replaced exclusively from below the upper core plate is disclosed in U.S. Pat. No. 5,297,176 titled "Remotely Replaceable Fuel Assembly Alignment Pin" issued Mar. 22, 1994 in the name of David A. Altman, et al. and assigned to the assignee of the present invention. Although the Altman, et al. patent discloses a guide pin for aligning a top nozzle of a nuclear fuel assembly to an upper core plate and is replaceable by working entirely from below the upper core plate, this patent does not disclose an alignment pin obviating the time-consuming need to drill-out the alignment pin.
Therefore, what is needed is a self-locking alignment pin and method for aligning a nuclear fuel assembly with respect to a core plate disposed in a nuclear reactor pressure vessel, the alignment pin being connectable to the core plate exclusively from below the core plate.